The present invention relates to catalytic cracking and an improved catalyst for use therein, particularly high alumina content catalyst, and particularly synthetic high alumina catalyst prepared by simultaneous cogellation of alumina and silica.
Catalytic cracking is a hydrocarbon conversion process in which a hydrocarbon, such as a petroleum oil fraction, is passed over a catalyst at an elevated temperature and at atmospheric or somewhat higher pressures to split higher molecular weight hydrocarbons into lower molecular weight substances, the principal end product being gasoline. The cracking temperatures are usually around 600.degree. to 1,000.degree. F., preferably around 900.degree. F. The cracking catalyst is typically circulated between the catalytic cracking reaction zone and the catalyst regeneration zone or vessel. The cracking catalyst is regenerated in the regeneration zone by burning deposited carbonaceous material off the catalyst at about 900.degree. to 2,000.degree. F., then the regenerated catalyst is recirculated to the catalytic cracking reaction zone. Steam is formed in the regeneration step because the carbonaceous material is combusted to carbon oxides and water vapor.
Typical commercial catalytic cracking data is shown by the table below:
TABLE 1 __________________________________________________________________________ COMMERCIAL CATALYTIC CRACKING DATA Operating Conditions Catalyst Synthetic beads, 15% Al.sub.2 O.sub.3 85% SiO.sub.2, 0.003% Cr __________________________________________________________________________ Reactor pressure 10 psig Temperature of oil vapor feed 783.degree.F. Temperature of catalyst to reactor 900.degree.F. Catalyst to oil ratio 3.8 vol/vol Carbon on spent catalyst 1.12% wt Carbon on regenerated catalyst 0.04% wt Feed space velocity 1.7 LHSV* Products, Weight % of Feed H.sub.2, CH.sub.4, C.sub.2 H.sub.4, C.sub.2 H.sub.6 1.1 C.sub.3 H.sub.6 1.5 C.sub.3 H.sub.8 1.2 C.sub.4 H.sub.8 2.1 i--C.sub.4 H.sub.10 3.7 n--C.sub.4 H.sub.10 0.9 Gasoline, 33-202.degree.C 32.3 Naphtha, 188-231.degree.C 4.7 Fuel oil, 218-338.degree.C 38.8 Heavy oil, 18.3.degree.API 10.5 Coke 3.2 100.0 __________________________________________________________________________ *LHSV = Liquid hourly space velocity, volumes of liquid feed per volume o catalyst per hour.
As can be seen from the above data, the amount of C.sub.3 and C.sub.4 olefins typically produced is about four weight per cent. The Table 1 data is for a plant using TCC beads; the olefin yield from a plant using typical fluidized catalyst produced by spray drying would be about 5 weight per cent.
Various methods are known for the preparation of catalytic cracking catalysts such as the commercial methods described by Ryland et al. starting at page 6 in Emmett's "Catalysis" Volume 7, Reinhold Publishing, 1960. Frequently, synthetic silica-alumina cracking catalysts are prepared by neutralizing an alkali metal silicate solution with a mineral acid, adding alum solution to the resulting hydrogel to incorporate the requisite amount of alumina in the final catalyst and precipitating the alum as alumina by the addition of a suitable base. Then the catalyst material is washed, spray dried (or formed as a bead in oil) and calcined.
As mentioned by Ryland et al, at page 9, there are numerous other methods for preparing alumina-silica catalysts including gellation of the desirable solubilized constituents under controlled conditions.
Natural clay material can be used as cracking catalysts after suitable treatment, such as acid activation. As indicated by one of the references reviewed by Ryland et al. at page 16, it is believed that the cracking activity of clay catalysts is not primarily due to the crystalline phase of the clay material but rather to an amorphous phase of the clay formed by the acid treatment. Also, it is believed that the amorphous phase of the clay formed by the acid treatment is essentially identical with a synthetic alumina-silica catalyst which in turn is believed to consist of a mixture of extremely small alumina and silica particles sharing oxygens or in intimate contact.
In many instances, because of the lower cost of clay materials, clay materials are used as a relatively inactive portion of synthetically prepared silica-alumina catalysts. That is, the clay can be used to "extend" the more expensive synthetically prepared silica-alumina. As pointed out by H. H. Voge at page 408 and 417 in Emmett's "Catalysis" Volume 6, Reinhold Publishing, 1958, a synthetic alumina-silica cracking catalyst ordinarily contains 10-25 weight per cent alumina. Ryland et al. at page 6 points out that early synthetic alumina-silica catalysts contained about 10 -12 per cent alumina, but that later high alumina catalysts were made containing about 25 per cent alumina.
An article by B. H. Loper, "Oil and Gas Journal," Apr. 25, 1955, page 115, also describes commercial results from "high alumina" catalysts which contain 25 weight per cent alumina.
The following patents disclose cracking catalysts of high alumina content:
U.S. Pat. No. 2,469,314 (Ryland et al.) 25 weight per cent alumina.
U.S. Pat. No. 3,023,172 (Innes et al.) 25 wt. per cent alumina exclusive of added low cost kaolin clay.
U.S. Pat. No. 3,010,914 (Braithwaite et al.) discloses a cracking catalyst composed of synthetic alumina-silica and added clay with the portion of alumina in the synthetic alumina-silica being 15 -80 wt. per cent alumina. The clay, in general, is 20-85 wt. per cent of the final catalyst. As per example 1 of the patent, the catalyst can be made by adding aqueous acid to a solution containing sodium aluminate and kaolin clay, and then adding a solution of alum and water so that the resulting pH of the slurry is 9.5. The resulting alumina-silica composition contains about 72.5 per cent alumina and 27 per cent silica, presumably based on the entire composition which includes the kaolin clay.
U.S. Pat. No. 3,010,914 also mentions that the activity of the catalyst measured after steaming of the catalyst is of more practical value in evaluating the catalyst than the activity without steam pretreatment.
Although U.S. Pat. No. 3,010,914 discloses relatively high alumina contents for a synthetic silica-alumina catalystcontaining clay, a subsequent Braithwaite et al. patent, U.S. Pat. No. 3,034,995, suggests lower alumina contents for synthetic silica-alumina catalyst-containing clay. Thus, according to U.S. Pat. No. 3,034,995, the synthetically dry silica-alumina hydrogel portion of the clay-containing catalyst contains 5 to 45 weight per cent alumina and 55 to 95 weight per cent silica.
U.S. Pat. No. 3,131,156 (Wilson et al.) is also directed to high alumina catalyst containing up to about 30 weight per cent alumina.
Current commercial cracking catalysts have a wide range of alumina contents. For example, commercial Catalyst A (believed to contain clay) 55 weight per cent alumina; commercial Catalyst B (also believed to contain clay) 50.5 weight per cent alumina; commercial Catalyst C, 46 weight per cent alumina; commercial Catalyst D, 35 weight per cent alumina, 65 weight per cent silica in the synthetic alumina-silica matrix, but the finished catalyst also includes added crystalline zeolites (aluminosilicate molecular sieves); Catalyst E, stated to be a high alumina catalyst, 25 weight per cent alumina; Catalyst F, stated to be "low alumina", 13 wt. per cent alumina; Catalyst G, stated to be a high alumina, 25 wt. per cent alumina; Catalyst H, 13 wt. per cent alumina; Catalyst I, 10 wt. per cent alumina.
By hindsight, after carrying out our experimental work, one reference which we found to be particularly pertinent is a C. L. Thomas article, at page 2564, "Industrial and Engineering Chemistry," Volume 41, No. 11. This article discloses a peak in cracking activity at alumina to silica ratios which approach the alumina content required in the synthetic alumina-silica matrix of the cracking catalyst to which the present specification is directed. In addition to the necessity to take the Thomas reference with the other prior art as a whole, we also note that the Thomas reference does not point out whether or not the catalysts tested by Thomas were steamed prior to testing. FIG. 1 from the Thomas reference is presented in revised form as part of FIG. 1 of the present specification and is discussed further hereinbelow.